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How to choose the right hydraulic fluid?

When it’s time to change or refill the hydraulic fluid in your system, it’s absolutely crucial that you make the right choice if you want to be able to trust your system and ensure it works efficiently. However, a lot of people feel unsure about how to choose from all the various hydraulic fluids for fear of unexpected stoppages and breakdowns. We know that choosing the right fluid for your hydraulic system can feel quite complex, but it doesn’t have to be that hard.

Start off by seeing if you have any documentation showing the required specifications of the hydraulic fluid – and if you have, that simplifies the whole process a lot. If you don’t have that information, you need to consider where and how the hydraulic fluid is used. For instance, is it used indoors? Or is it exposed to temperature fluctuations outdoors? If you want to use an environmentally classified product, you should use a hydraulic fluid based on synthetic esters. A lot of companies would benefit from doing this, as nowadays they often bring energy savings with less of an environmental footprint, as well as better technical properties than conventional mineral oils.

Feel free to use our Oil Guide to find the right oil for the right machine.

Viscosity and Mixing hydraulic fluids

Be aware of what viscosity is needed in your system. Viscosity is about how ‘thick’ a fluid is, and it is measured on the ISO VG scale (International Standards Organization Viscosity Grade). Hydraulic fluids often have ISO VG 32, ISO VG 46 or ISO VG 68, but there are many more viscosity grades to choose from. The higher the figure, the thicker and more viscous the fluid – and of course vice versa, so the lower the figure, the thinner and more easily flowing.

Viscosity is very much affected by temperature. Generally speaking a lower viscosity is used in low temperatures and a higher one in higher temperatures, since oil naturally becomes thinner when hot and thicker when cold. A hydraulic system that struggles in the cold probably doesn’t have the optimum fluid viscosity for the operating temperature. To clarify how the viscosity of a hydraulic fluid changes with temperature, a Viscosity Index or VI is used. The VI defines the oil’s ability to resist changes in viscosity when the temperature changes: the higher the VI, the better the oil’s viscosity stability as the temperature fluctuates. So a product with a high VI will retain its viscosity and flow better also when the temperature falls, while a low VI product will become more viscous in lower temperatures.

Generally speaking, hydraulic fluids for outdoor use should have a VI of 150 or higher. This means the product will work within a broad temperature range, which safeguards operation whether it’s warm or cold. Modern hydraulic systems work under high pressures and heavy flows, which places high demands on the hydraulic fluid’s ability to maintain the right thickness and to separate air and water. Having said that, a high VI is less important in industrial hydraulic systems, for example, which tend to operate in consistent temperature ranges and generally require different key properties.

As far as possible, it is always best to avoid mixing different hydraulic fluids. This is because the technical properties could be spoiled by chemical reactions between different additives. As a result, the additives could work against rather than with each other, which could eliminate the original properties of the fluids or change them completely. If you are unsure which product was originally used to fill the hydraulic system, it may therefore be wise to drain and change the oil rather than topping it up with another product.

If you need help finding out whether two hydraulic oils are miscible, ask your lubricant supplier, who will be able to give you a sure answer by conducting a miscibility test.

Did you know that the right hydraulic oil decreases energy loss?

Therefore, choose an oil with the right properties, especially in terms of viscsity. An oil with lower viscosity is easier to "pull" around the hydraulic system, thus reducing energy loss and saving money. Many people choose a high-viscosity oil out of habit, but this is an area worth reflecting on to achieve improved lubrication and lower costs.